1. Field of the Invention
This invention relates to a system such as an Internet Protocol (IP) telephone system which performs voice communication between telephone terminals via an IP network and more particularly to a communication system which performs communication between a telephone terminal connected to a global network and a telephone terminal connected to a private network, and a communication terminal device used in the communication system and a communication method used in the communication system.
2. Description of the Related Art
Recently, a network telephone system (IP telephone system) which bi-directionally transmits/receives images and voice as packet data on a real-time basis via a packet network has been adopted. In the IP telephone system, a plurality of IP telephone terminals are provided in a private network such as a LAN (Local Area Network) and the private network is connected to a global network such as a public communication network via a router device. Thus, multimedia information communication can be performed between the IP telephone terminals and between the JP telephone terminal and the global network by performing protocol conversion, address conversion and the like in the router device.
In the above system, a Network Address Translator (NAT) function is provided on the router device. The NAT function is a function of converting a private address into a global address and transmitting the same as a transmission-source IP address by use of one IP address previously assigned for connection, and converting a global address of a destination IP address supplied in the reception mode into a private address. The NAT function is specified by RFC (Request For Comments) 1631 (for example, refer to Japanese Patent Application KOKAI Publication No. 2003-289318, Japanese Patent Application KOKAI Publication No. 2002-368773 and Japanese Patent Application KOKAI Publication No. 2003-324482).
In the above system, address conversion is performed on an IP address and a port number included in a header area of an IP packet in a router device in communication in which a media channel different from the control channel is newly opened at the time of communication. However, address conversion is not performed by the router device on an IP address and a port number included in a data area of the IP packet. Therefore, even if an attempt is made to answer voice data to a transmission-source of the IP packet by use of an IP address and a port number included in the data area of the IP packet by a terminal which receives the IP packet, the destination cannot be correctly specified, when a terminal on the private network communicates with a terminal on the global network via a router that provides the NAT function.